Handout 2: Structure and Function of the Reproductive
System of the Bull, Boar and Stallion

General Structure and Function

A. The Testes. These primary organs have the dual function of
producing sperm cells, and also the male hormone, testosterone. The
testis is enclosed in the tunica albuginea surrounded by another tough tunic,
the tunica vaginalis. The seminiferous tubules are the site of sperm formation.
These in turn empty into collecting ducts, the rete testis, lined with cuboidal
epithelium. The supporting connective tissue joins centrally forming a fibrous
cord, the mediastinum testis.

B. The secondary sex organs are the ducts and tubes which
convey the sperm cells out of the testes and eventually out of the body.
They are listed in order in which sperm pass through them.

Vas Efferentia (ductuli efferentes) - about 12 ducts arising
from the rete testis, emerging from the testis and emptying into the epididymis.

Epididymis - A large tortuous duct outside the testis through
which the sperm migrate slowly. It is divided into the head, body and tail.
The tail portion is the chief site of sperm storage.

Vas Deferens (ductus deferens) - A slender tube connecting the
epididymis with the urethra and which enlarges into an ampulla just before
joining the urethra. (Often called vas deferens, when cut - vasectomy.)

Urethra - The tube in the penis through which semen is discharged
at the time of copulation, and through which urine is excreted.

The general location of the different parts of the reproductive tract
of the bull is shown in the following diagram.

While it is possible to show only 1 testis clearly in this diagram there
are two testes, and two sets of ducts carrying the sperm to the urethra.
The urethra is a single duct, which also carries urine from the bladder.

The testes are oval-shaped organs 4 to 5 inches in length, 2-1/2 to 3
inches in diameter, with the long axis being vertical. Each testis weighs
10 to 12 ounces in a mature bull. They lie outside the body cavity in a
pouch of skin called the scrotum. An important purpose of the scrotum is
to provide the testes with an environment which is a few degrees (2-8°C)
cooler than body temperature. This cooler temperature is necessary for the
formation of spermatozoa. Failure of the testes to descend from the abdomen
into the scrotum, associated normally with shortening of the gubernaculum
and intra-abdominal pressure, results in a condition known as cryptorchidism.
This will cause sterility if both testes fail to descend (bilateral cryptorchidism).
Unilateral cryptorchid animals may be fertile, but it is thought that this
condition may be inherited, and breeding males possessing this trait should
be avoided.

For many years the external cremaster muscle within the spermatic cord
has been thought of as the principle thermoregulator of the scrotum, drawing
the testes close to the abdomen when cold and relaxing when warm. However,
the tunica dartos muscle at the bottom of the scrotum also responds to temperature
changes and probably plays a major role in temperature regulation of the
testes. It has been shown that this latter muscle is sensitive to temperature
changes only in the presence of the male hormone, testosterone. Blood flowing
to the testis is cooled by adjacent venous return in a convoluted complex
of vessels called the pampiniform plexus, located just dorsal to the testes.

The testes are partially supported by the spermatic cord, which runs
from the abdomen and is attached to the testes in the scrotum. This band
of tissue, carrying the ductus deferens, blood vessels, nerves and muscles
associated with the testis, may be 8 to 10 inches or more in length.

The many convoluted seminiferous tubules in the testis in which the spermatozoa
are formed finally straighten and join to form the rete testis. Arising
from the rete testis are 12 or more out-going ducts, the vas efferentia,
which emerge from the testis and enter the epididymis. The epididymis is
a single large tortuous tubule lying on the surface of the testis. Its purpose
is to collect and store the sperm while the latter undergo a ripening process.
The different parts of the epididymis are referred to as the head, the body,
and the tail (caput, corpus and cauda). It is the tail of the epididymis
that the majority of sperm are stored. The relationships of these structures
are shown in Fig. 2.

From the epididymis, the spermatozoa are carried into the vas deferens,
the tube which carries the testicular products to the urethra of the penis.
The vas deferens are very small in diameter, having a cartilaginous cord-like
appearance, and each forms a saccular ampulla, 4 to 5 inches long and 1/2
inch in diameter, near the junction with the pelvic urethra. The two ampullae
empty into the urethra. The urethra serves as the common passage for urine,
and upon ejaculation for semen.

The organ of copulation is the penis. In the adult bull it is about 3
feet in length and about 1 inch in diameter. There is very little erectile
tissue present except in the root. Even in the relaxed state the penis is
very dense and firm. Behind the scrotum it forms an S-shaped curve, the
sigmoid flexure. During erection this flexure is straightened out, thus
increasing the length of the organ. Also, at the time of erection the bulbospongiosus
and ischiocavernosus muscles at the root of the penis contract and assist
in causing an erection and in ejecting the semen. The retractor penis muscle
assists in withdrawing the penis into the sheath after copulation. The sheath
is long and narrow. The preputial opening is a short distance behind the
navel and is usually surrounded by long hairs.

The accessory glands associated with the reproductive tract, secrete
a large part of the semen. In most species, these are the seminal vesicles,
the prostate, and the bulbo-urethal or Cowper's glands (see Fig. 1). The
paired seminal vesicles are tortuous lobulated glands, which produce a large
volume of fluid which may flush the urethra and act as a vehicle for sperm
transport. This secretion also includes fructose. The prostate is a compound
gland lying over the urethra at the neck of the bladder. It produces a complex
secretion, which stimulates sperm activity. The paired Cowper's glands lie
below the prostate on either side of the urethra, and produce a viscid mucus-like
lubricating substance. The secretions of all of the accessory glands plus
the spermatozoa and the secretions of the testes and tubes leading from
it make up the semen. These secretions aid in the nutrition of the sperm
and provide some buffering capacity.

Cross-Section of the Testis

If one takes thin slices from the testis and examines them with a microscope
at high magnification the testis is seen to be full of tiny tubules, the
seminiferous tubules. These tightly coiled tubules, thousands of yards in
total length, are lined with germinal epithelium, from which the sperm cells
are eventually formed. In between the tubules is found the interstitial
tissue. This tissue produces the male hormone, testosterone. Also, this
tissue gives some support to the testis. The cut seminiferous tubules and
interstitial tissue are shown diagrammatically in Fig. 3.

Reproductive System of the Boar

The preceding information on the bull applies in a general way to the
boar. The names of the different parts of the reproductive system are the
same. However, the relative sizes and arrangements of the various parts
may differ from the bull as shown in Fig. 4. The testes of the boar are
relatively large. They are suspended in an inverted position (compared to
the bull and ram) with the tail of the epididymis uppermost. Particularly
striking are the large Cowper's glands. These are the source of the gelatinous
material in boar semen. Also, note the rotation of the testis relative to
the vertical position in the bull. The penis has no true glans at the tip.
The preputial pouch is pronounced in the boar. This contains some residual
urine and decomposing epithelial cells which contribute to the unpleasant
odor which may permeate boar meat.

Figure 4. The Boar.

Reproductive System of the Horse

The reproductive system of the stallion is shown in Fig. 5. Several differences
from the bull and boar are distinctive. In the relaxed state the testes
are nearly horizontal. They vary greatly in size depending upon the breed.
The body of the epididymis is large. The penis is a vascular-muscular type
and so enlarges greatly during erection. Considerable smegma accumulates
in the preputial cavity and the penis should be washed before collecting
semen or before copulation. There is no sigmoid flexure.

The Cowper's glands (bulbo-urethral glands) are about 1-2 inches in diameter
and can be palpated rectally, in contrast with the bull. The seminal vesicles
and prostate also can be palpated rectally near the junction of the bladder
and the urethra.